U.S. patent application number 14/285094 was filed with the patent office on 2014-11-13 for microlithographic projection exposure apparatus.
This patent application is currently assigned to Carl Zeiss SMT GmbH. The applicant listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Florian Bach, Daniel Benz, Sascha Bleidistel, Yim-Bun Patrick Kwan, Severin Waldis, Armin Werber.
Application Number | 20140333912 14/285094 |
Document ID | / |
Family ID | 41719875 |
Filed Date | 2014-11-13 |
United States Patent
Application |
20140333912 |
Kind Code |
A1 |
Bleidistel; Sascha ; et
al. |
November 13, 2014 |
MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS
Abstract
A microlithographic projection exposure apparatus has a mirror
array having a base body and a plurality of mirror units. Each
mirror unit includes a mirror and a solid-state articulation, which
has at least one articulation part that connects the mirror to the
base body. A control device makes it possible to modify the
alignment of the respective mirror relative to the base body.
Mutually opposing surfaces of the mirror and of the base body, or
of a mirror support body connected to it, are designed as
corresponding glide surfaces of a sliding bearing.
Inventors: |
Bleidistel; Sascha; (Aalen,
DE) ; Kwan; Yim-Bun Patrick; (Aalen, DE) ;
Bach; Florian; (Oberkochen, DE) ; Benz; Daniel;
(Winnenden, DE) ; Waldis; Severin; (Bern, CH)
; Werber; Armin; (Gottenheim, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
|
DE |
|
|
Assignee: |
Carl Zeiss SMT GmbH
Oberkochen
DE
|
Family ID: |
41719875 |
Appl. No.: |
14/285094 |
Filed: |
May 22, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13052265 |
Mar 21, 2011 |
8767176 |
|
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14285094 |
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PCT/EP2009/006718 |
Sep 17, 2009 |
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13052265 |
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61101281 |
Sep 30, 2008 |
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Current U.S.
Class: |
355/67 |
Current CPC
Class: |
G03F 7/70291 20130101;
G02B 26/0833 20130101; G02B 17/002 20130101; G03F 7/70116 20130101;
G02B 7/1827 20130101; G03F 7/702 20130101; G03F 7/70891
20130101 |
Class at
Publication: |
355/67 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G02B 17/00 20060101 G02B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2008 |
DE |
10 2008 049 556.5 |
Claims
1-21. (canceled)
22. An optical system, comprising: a mirror array, comprising: a
base body; a plurality of mirror units supported by the base body,
each mirror unit comprising: a mirror; a solid-state articulation
comprising first and second articulation parts, each articulation
part connecting the mirror to the base body, each articulation part
being capable of bending in a plane of bending, and each
articulation part being subdivided into a plurality of articulation
elements that are spaced apart from each other in the plane of
bending to reduce a flexural stiffness of the at least two
articulation parts; and a control device configured to modify an
orientation of the mirror relative to the base body, wherein the
optical system is a projection objective of a microlithographic
projection exposure apparatus or an illumination system of a
microlithographic projection exposure apparatus.
23. The optical system of claim 22, wherein, for each mirror unit,
the articulation elements of each of the first and second
articulation parts extend at least substantially parallel to each
other.
24. The optical system of claim 22, wherein the articulation
elements comprise rods.
25. The optical system of claim 22, wherein the articulation
elements comprise plates.
26. The optical system of claim 22, wherein the first and second
articulation parts are arranged on opposite sides of a plane of
symmetry of the mirror.
27. The optical system of claim 26, wherein the first and second
articulation parts are arranged mirror symmetrically with respect
to the plane of symmetry of the mirror.
28. The optical system of claim 22, wherein each mirror unit
comprises flexible thermal conduction elements extending between
the mirror and the base body.
29. The optical system of claim 22, wherein at least a portion of
the control device is arranged between the first and second
articulation parts.
30. The optical system of claim 22, further comprising a third
articulation part between first and second articulation parts,
wherein the third articulation part is shorter, but thicker than
the articulation elements.
31. The optical system of claim 22, wherein the mirror is
configured to swivel about a swivel axis that lies at least
substantially in a reflection surface of the mirror.
32. The optical system of claim 22, wherein each mirror unit
comprises a sensor device configured to determine the orientation
of the mirror with respect to the base body.
33. The optical system of claim 22, wherein the mirror is
configured to reflect light having a wavelength shorter than 25
nm.
34. The optical system of claim 33, wherein the mirror is
configured to reflect light having a wavelength of about 13.5
nm.
35. An apparatus, comprising: the optical system of claim 22,
wherein the apparatus is a projection exposure apparatus.
36. The apparatus of claim 35, further comprising a light source
configured to produce light having a wavelength shorter than 25
nm.
37. The projection exposure apparatus of claim 36, wherein the
light source is configured to produce light having a wavelength of
about 13.5 nm.
38. An optical system, comprising: a mirror; a base body; and a
solid-state articulation connecting the mirror to the base body,
wherein: the solid-state articulation is subdivided into a
plurality of mutually parallel articulation elements that are
rod-shaped or plate-shaped; the articulation elements are spaced
apart from each other in a plane of bending to reduce a flexural
stiffness of the solid-state articulation; and the optical system
is a projection objective of a microlithographic projection
exposure apparatus or an illumination system of a microlithographic
projection exposure apparatus.
39. A method, comprising: providing a mirror array comprising a
base body and a plurality of mirror units supported by the base
body, each mirror unit comprising a mirror and an articulation part
that connects the mirror to the base body, determining a target
flexural stiffness that for the articulation part; determining a
target thermal conductivity that for the articulation part;
determining a total cross section that the articulation part should
have to achieve the target thermal conductivity; determining a
number of mutually separated articulation elements which define the
articulation part so that an entirety of all articulation elements
has the target flexural stiffness and the total cross section.
40. The method of claim 39, wherein the articulation elements
defining the articulation part extend at least substantially
parallel to each other.
41. The method of claim 39, wherein the articulation elements
comprise rods.
42. The method of claim 39, wherein the articulation elements
comprise plates.
43. The method of claim 39, wherein each mirror unit comprises a
control device configured to modify an orientation of the mirror
relative to the base body.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims benefit
under 35 USC 120 to, international application PCT/EP2009/006718,
filed Sep. 17, 2009, which claims benefit of German Application No.
10 2008 049 556.5, filed Sep. 30, 2008 and U.S. Ser. No.
61/101,281, filed Sep. 30, 2008. International application
PCT/EP2009/006718 is hereby incorporated by reference in its
entirety.
FIELD
[0002] The disclosure relates to microlithographic projection
exposure apparatus, and in particular to illumination systems or
projection objectives of such apparatus having a mirror array,
which has a base body and a plurality of mirrors that are arranged
on the base body and can be tilted or otherwise modified in their
alignment relative to the base body.
BACKGROUND
[0003] Integrated electrical circuits and other microstructured
components are conventionally produced by applying a plurality of
structured layers onto a suitable substrate which, for example, may
be a silicon wafer. In order to structure the layers, they are
first covered with a photoresist which is sensitive to light of a
particular wavelength range, for example light in the deep
ultraviolet (DUV) or extreme ultraviolet (EUV) spectral ranges.
Conventional light wavelengths for DUV systems are currently 248
nm, 193 nm and sometimes 157 nm; EUV projection exposure apparatus
currently use X-ray light with a wavelength of about 13.5 nm.
[0004] The wafer coated in this way is subsequently exposed in a
projection exposure apparatus. A pattern of structures, which is
arranged on a mask, is thereby imaged onto the photoresist with the
aid of a projection objective. Since the imaging scale is generally
less than 1, such projection objectives are often also referred to
as reducing objectives.
[0005] After the photoresist has been developed, the wafer is
subjected to an etching process so that the layer becomes
structured according to the pattern on the mask. The photoresist
still remaining is then removed from the other parts of the layer.
This process is repeated until all the layers have been applied
onto the wafer.
[0006] The performance of the projection exposure apparatus used is
determined not only by the imaging properties of the projection
objective, but also by an illumination system which illuminates the
mask. To this end, the illumination system contains a light source,
for example a laser operated in pulsed mode (DUV) or a plasma
source (EUV), and a plurality of optical elements which generate
light beams, converging on the mask at field points, from the light
generated by the light source. The individual light beams desirably
have particular properties, which in general are adapted to the
projection objective and the mask to be imaged.
[0007] In order to be able to vary more flexibly the properties of
the light beams striking the mask or the shape of the region
illuminated on the mask, it has been proposed to use one or more
mirror arrays, each having a plurality of adjustable mirrors, in
the illumination system. The alignment of such mirrors is
conventionally carried out by swiveling movements about one or two
swivel axes. Such swiveling mirrors are therefore desirably fitted
to suspensions which have one or two movement degrees of freedom.
This may, for example, be achieved with solid-state articulations
or with universal suspensions.
[0008] Mirror arrays, each having a plurality of adjustable
mirrors, may also be used in projection objectives. For example, an
array in a pupil plane of the projection objective may be envisaged
in order to correct particular field-independent imaging
errors.
[0009] The reflective layer systems, which are applied onto the
supports of the adjustable mirrors, absorb an (albeit small) part
of the incident light even in DUV projection exposure apparatus. In
EUV projection exposure apparatus, the losses due to absorption are
about 30%. The light absorbed by the mirrors heats them and, if
sufficient dissipation of heat is not ensured, can lead to
destruction of the reflective layer systems or other parts of the
mirror units.
SUMMARY
[0010] The disclosure provides projection exposure apparatus having
mirror arrays, in which the heat produced in the mirrors is
dissipated particularly well so that overheating can reliably be
avoided.
Subdivided Solid-State Articulation
[0011] According to a first aspect of the disclosure, a
microlithographic projection exposure apparatus has a mirror array
having a base body and a plurality of mirror units. Each mirror
unit includes a mirror and a solid-state articulation, which has at
least one articulation part that connects the mirror to the base
body and is capable of bending in a plane of bending. A control
device makes it possible to modify the alignment of the respective
mirror relative to the base body. According to a first aspect of
the disclosure, the articulation part is subdivided into a
plurality of articulation elements that are spaced apart from each
other in the plane of bending in order to reduce the flexural
stiffness of the articulation part.
[0012] The spacing between the articulation elements may be very
small. Small gaps between adjacent articulation elements may be
filled with a liquid or a gas. The spacings may be so small that
adjacent articulation elements even slightly touch each other. The
plane of bending is generally arranged perpendicularly to a swivel
axis around which the mirror is allowed to swivel.
[0013] This aspect of the disclosure is based on the idea that when
solid-state articulations are used, components through which heat
can be dissipated from the mirror by thermal conduction are already
available. Utilising the solid-state articulation to dissipate heat
from the mirrors is advantageous because, unlike other types of
articulation, solid-state articulations do not have any gas- or
liquid-filled gaps which can impede the heat transfer. The
articulation parts of the solid-state articulation, however,
generally have a very filigree design since otherwise the desired
flexural properties cannot be achieved.
[0014] Owing to the inventive subdivision of the articulation parts
into a plurality of mutually separated smaller articulation parts,
it is possible to increase the articulation part's total cross
section available for the heat flux without significantly modifying
the flexural properties. This uses the effect that is known from
subdividing a rod into a plurality of thin sub-rods that are spaced
apart from each other in the plane of bending. Subdividing the rod
then reduces its bending strength. If the bending strength is
intended to remain constant after the subdivision, then additional
sub-rods desirably are added, so that the total cross section and
therefore the transferable heat flux are increased.
[0015] In one embodiment, the articulation elements are at least
essentially arranged mutually parallel. Often, however, deviations
from parallel are expedient so that the forces acting on the
individual articulation elements can be adapted better to one
another.
[0016] In another embodiment, the articulation elements are
rod-shaped or plate-shaped. Rods and plates have well-defined
flexural properties and are therefore particularly suitable as
articulation elements.
[0017] In order to construct a solid-state articulation, two
articulation elements may engage on the mirror while being mutually
opposite. The mirror can then be swiveled in both directions about
a swivel axis, which is established by the engagement points of the
articulation elements.
[0018] The disclosure provides a method for developing a
microlithographic projection exposure apparatus which has a mirror
array that includes a base body and a plurality of mirror units,
each of which has a mirror, a solid-state articulation which has at
least one articulation part that connects the mirror to the base
body, and a control device by which the alignment of the respective
mirror relative to the base body can be modified. The method
includes: [0019] i) establishing a flexural stiffness which the
articulation part should have; [0020] ii) establishing a thermal
conductivity which the articulation part should have; [0021] iii)
establishing a total cross section, which the articulation part
should have in order to achieve the thermal conductivity
established in step ii); [0022] iv) establishing a number of
mutually separated articulation elements which form the
articulation part, such that the total set of articulation elements
has the flexural stiffness established in step i) and the total
cross section established in step iii).
Additional Thermal Conduction Elements
[0023] According to another aspect of the disclosure, a
microlithographic projection exposure apparatus has a mirror array
having a base body and a plurality of mirror units. Each mirror
unit includes a mirror and a control device, by which the alignment
of the respective mirror relative to the base body can be modified.
According to the disclosure, the mirror units have thermal
conduction elements which do not contribute to the bearing of the
mirror, which are connected to the mirrors and which extend in the
direction of the base body so that heat can be transferred from the
thermal conduction elements to the base body.
[0024] This aspect of the disclosure is based on the idea that the
heat transport from the mirror to the base body can be improved
with the aid of additional thermal conduction elements, which are
not part of the articulation.
[0025] The greatest heat flux is achieved when the thermal
conduction elements are connected to the base body. In this case,
the thermal conduction elements may be designed as flexible fibres
or flexible bands which have a vanishingly small bending strength
and therefore do not impede swiveling movements of the mirror. With
a sufficiently large number of such thermal conduction elements,
for example several hundred, it is nevertheless possible to provide
a considerable total cross section overall through which the heat
flux can pass.
[0026] It is however also possible not to connect the thermal
conduction elements to the base body, so that the heat also travels
through a preferably maximally small gap which is filled with a
fluid or through which a fluid flows, in order to reach the base
body. For example, it is conceivable to design the thermal
conduction elements as essentially rigid bars. At least essentially
rigid counter-elements may then protrude from the base body, which
are separated from the thermal conduction elements only by a gap
even during modifications of the alignment of the mirror.
[0027] As already mentioned, the gap width should be as small as
possible since solids generally have a higher conductivity for heat
than gases do. This applies particularly when the gas pressure is
very low, as is desirable for EUV projection exposure apparatus. In
these cases, the gap should have a gap width which is less than
1/10 of the maximum dimension of a reflective surface of the
mirror.
[0028] In another embodiment, the bars and the counter-elements are
arranged on the mirror and on the base body, respectively, so that
they mutually engage in a comb-like fashion. Such an arrangement is
advantageous because, overall, it provides a large surface area
through which the heat can be transferred from the bars to the
counter-elements.
[0029] If the bars and the counter-elements are configured in the
form of cylinder wall segments and are arranged concentrically,
then the gap width can remain constant even when the mirror is
swiveled relative to the base body.
[0030] In particular silicon, a silicon compound, particularly
silicon carbide, carbon or a metal, particularly copper, silver or
gold may be envisaged as materials for the thermal conduction
elements. These materials have a particularly high thermal
conductivity and can also be processed well in precision mechanical
applications.
[0031] The bars may also be used to provide an electrostatic drive,
if the control device has a voltage source by which the bars can be
electrostatically charged.
Fluidic Cooling
[0032] According to another aspect of the disclosure, a
microlithographic projection exposure apparatus includes a mirror
array. The mirror array has a base body and a plurality of mirror
units, each of which has a mirror and a control device, by which
the alignment of the respective mirror relative to the base body
can be modified. According to the disclosure, the mirror units
respectively have a flexible sealing mechanism which hermetically
delimit a volume section between the mirror and the base body.
[0033] This aspect of the disclosure is based on the idea that,
particularly in EUV projection exposure apparatus, it is not
possible to select the pressure of the gas surrounding the mirror
at such a high level that the gas can make a significant
contribution to cooling the mirror. Immersing the mirrors in
liquids is problematic, even in DUV projection exposure
apparatus.
[0034] Yet by inventively providing a volume section between the
mirror and the base body, which is delimited hermetically by the
flexible sealing mechanism, this volume section can be filled with
a gas or a liquid, or a gas or a liquid may flow through it, so as
to make a significant contribution to cooling the mirrors.
[0035] In the simplest case, the volume section is filled once or
at long time intervals with the liquid or gaseous fluid, which
remains there. The heat flux is then provided essentially by
thermal conduction in the stationery fluid.
[0036] An even higher cooling power will be achieved if the fluid
is circulated in the volume section, so that the heat transport
takes place primarily by convection. To this end, the volume
section may have an inlet and an outlet. A circulation device,
which may for example contain a pump and a heat exchanger, will be
allocated to the mirror unit in order to circulate the fluid in the
volume section.
[0037] If the sealing mechanism includes flexible sealing strips,
which connect neighbouring mirrors to one another, then the
fluid-tight volume section may extend over the entire space below
the mirror and the rest of the sealing mechanism. The areas via
which the mirrors come in contact with the fluid, and can thereby
dissipate heat, will correspondingly be large.
[0038] If the fluid is a gas, it will preferably have a higher
pressure in the volume section than a gas which is present on the
other side of the sealing mechanism. This exploits the fact that
the thermal conductivity of gases increases strongly with an
increasing pressure. Increasing the gas pressure also has a
favourable effect on the cooling power in the case of heat
transport by convection.
[0039] In another embodiment, the sealing mechanism are bellows.
This mirror unit will preferably have two bellows for each degree
of freedom, which are arranged opposite one another. Symmetrical
force conditions will thereby be provided when swiveling the
mirror.
[0040] So that the bellows oppose swiveling with the least possible
resistance, they may be connected together so that they communicate
fluidically. In this context, for example, it may be feasible to
connect the two bellows together by a channel which extends through
the mirror.
[0041] If the control device has a device, in particular a
displaceable piston or a pump, for modifying the fluid volume
enclosed by a bellows, then it is also possible to use the bellows
as actuation mechanism by which the alignment of the mirror
relative to the base body can be modified. Additional actuators can
then be obviated.
Sliding Bearing
[0042] According to another aspect of the disclosure, a
microlithographic projection exposure apparatus includes a mirror
array. The mirror array has a base body and a plurality of mirror
units, each of which has a mirror and a control device, by which
the alignment of the respective mirror relative to the base body
can be modified. According to the disclosure, mutually opposing
surfaces of the mirror and of the base body, or of a mirror support
body connected to it, are designed as corresponding glide surfaces
of a sliding bearing.
[0043] This aspect of the disclosure is based on the idea that the
surfaces which bear on one another in sliding bearings are
relatively large compared with solid-state articulations, so that
the total achievable heat flux may be sufficient even if the heat
transfer through the gap between the glide surfaces is impeded.
[0044] This heat transfer can be increased if at least one glide
surface is provided with a lubricating and/or anti-wear coating, in
particular with a metallization or with a diamond coating. Such
coatings increase the contact area between the glide surfaces, and
therefore improve the heat transfer.
[0045] A significant improvement in the heat flux can be achieved
if a movement gap, which is at least partially filled with a paste
or a fluid, in particular with a liquid, is formed between the
corresponding glide surfaces. The liquid or paste prevents sizeable
gas-filled cavities, which restrict the heat flux, from being
formed in the movement gap.
[0046] Under certain circumstances, however, an increase in the
heat flux may also be achieved with a gas-filled movement gap. If
the gas is at a high pressure, for example, its thermal
conductivity increases significantly. Here, a high pressure
mechanism any pressure of more than 1.5 times the standard
operating pressure which prevails in a space of the apparatus,
through which projection light passes. If a gas flow is fed through
the movement gap, then it can dissipate heat convectively.
[0047] In an advantageous embodiment, the control device is
designed for variable adjustment of the width of the movement gap.
In this way, for example, it is possible to keep the movement gap
as small as possible and thereby increase the heat flux when the
mirror is at rest. If the mirror is intended to be swiveled, the
width of the movement gap will be increased shortly beforehand in
order to achieve better sliding properties. Depending on the type
of fluid contained in the movement gap, a similar effect may also
be achieved by variable adjustment of an application pressure
prevailing between the glide surfaces.
[0048] An additional improvement will be obtained if the glide
surface of the base body or of the mirror support body has at least
one exit opening for the fluid, from which the fluid can flow out
into the movement gap. A fluid flowing in the movement gap allows
additional dissipation of heat by convection.
[0049] In order to discharge the fluid again, the base body or the
mirror support body may have at least one entry opening for the
fluid, through which fluid circulating in the movement gap can flow
out of the movement gap.
[0050] By suitable delivery of the fluid in the movement gap, it is
possible to modify the flow direction of the fluid in the movement
gap. This may in turn be exploited in order to exert a torque on
the mirror and thereby swivel it.
[0051] Preferably, to this end the glide surface of the base body
or of the mirror support body has at least two exit openings for
the fluid, which lie diametrically opposite one another. Depending
on which of the openings the fluid emerges from, a swiveling
movement will be generated in one direction or the other.
[0052] In order to increase the torque which the fluid can exert on
the mirror, the glide surface of the mirror may be provided with
structures to increase the drag in relation to the fluid. Such
structures may, for example, be bars or grooves which extend
transversely to the flow direction.
[0053] A seal may furthermore be provided, which prevents the fluid
emerging from a gap that remains between the mirror and the mirror
support body.
[0054] The base body or the mirror support body may be prestressed
relative to the mirror.
[0055] It is to be understood that the aspects of the disclosure as
mentioned above may very substantially be combined with one
another. For example, irrespective of the type of bearing,
additional thermal conduction elements which extend from the mirror
to the base body may always be provided. Furthermore, it is
possible to provide fluidic cooling by using flexible sealing means
irrespective of the type of bearing.
[0056] Further variants, in which all the aspects of the disclosure
as described above may advantageously be employed, will be
described below.
[0057] Thus, the control device may for example include at least
one movement transducer arranged movably relative to the mirror, in
particular a piezo or ultrasonic motor.
[0058] The movement transducer may bear flat on a mirror section in
a resting state. In particular, the mirror and the movement
transducer may have corresponding contact surfaces in the form of
spherical cap segments. The sphere centres of the contact surfaces
may be arranged in or at least in the immediate vicinity of an
optical centre of the mirror. The term optical centre refers to the
region of the mirror which the light actually strikes during
operation of the apparatus. The optical centre therefore need not
necessarily coincide with the geometrical centre.
[0059] In this case, the mirror may be coupled to the base body via
a flexurally elastic and torsionally stiff spring element, in which
case the spring element may in particular be designed as a (metal)
bellows. The spring element may be elastically prestressed both in
a neutral setting and in an excursion setting of the mirror and
filled with a fluid, in particular with a liquid.
[0060] It is also preferable that the mirror can be swiveled about
a swivel axis, which lies in or at least approximately in a
reflection surface of the mirror. This will ensure that the
shadowing of the mirrors is minimal even in the event of a
swiveling movement of the mirror.
[0061] It is furthermore preferable that a sensor device for
determining the alignment of the mirror should be allocated to the
control device.
[0062] The disclosure may be used particularly advantageously when
the radiation source is adapted for generating light with a
wavelength shorter than 25 nm, in particular with a wavelength of
about 13.5 nm. With these wavelengths, the light desirably only
passes through volumes with very low gas pressures. Since gases
conduct heat only poorly at low pressures, the solutions according
to the disclosure have a particularly favourable effect on the
cooling of the mirrors.
[0063] The solutions described above may be used advantageously not
only in arrays having a plurality of mirror units, but also in
microlithographic projection exposure apparatus which contain an
adaptive mirror which can be deformed with the aid of a plurality
of control units (actuators). Such an adaptive mirror may be
regarded as an array of a plurality of mirror units, the individual
mirrors of which are connected together by reflective material
strips. The mirror units then consist essentially of the control
units, by which the shape of the (common) mirror can be modified.
The mirror array then corresponds to an array of control units
which has a base body and a plurality of control units fastened on
the base body, which engage on the mirror and by which the shape of
the mirror can be modified.
[0064] Furthermore, another possible field of application involves
projection exposure apparatus in which the total alignment of
larger and not necessarily adaptively adjustable individual mirrors
can be modified. For EUV projection objectives, for example
mirrors, have been proposed which are mounted magnetically and
therefore without any articulation which could contribute to
dissipation of heat by thermal conductivity. Most of the
aforementioned solutions may also be used advantageously for such
"floating", or at least in part not physically supported mirrors.
In the claims, the mirror array is then to be replaced with a
mirror and a control device, by which the alignment of the mirror
relative to a base body can be modified.
BRIEF DESCRIPTION OF THE DRAWINGS
[0065] Other features and advantages of the disclosure may be found
in the following description of preferred embodiments with the aid
of the drawing, in which:
[0066] FIG. 1 shows a perspective representation of a swivellable
mirror, which is suspended from a solid-state articulation formed
by a plurality of leaf springs and having one degree of freedom for
movement;
[0067] FIG. 2 shows a second embodiment of a swivellable mirror,
which is suspended from a solid-state articulation formed by a
plurality of leaf springs;
[0068] FIG. 3 shows a schematic representation of a swivellable
mirror, to which metal filaments are allocated for thermal
conduction;
[0069] FIG. 4 shows a schematic sectional representation of a
mirror with bars, which form an electrostatic drive, placed in
touching contact;
[0070] FIG. 5 shows adjacently arranged tiltable mirrors, which are
connected together by flexible sealing membranes;
[0071] FIG. 6 shows a swivellable mirror prestressed by a
prestressing device into an indentation in the form of a spherical
cap;
[0072] FIG. 7 shows a mirror cooled by gas on its rear side and
held in a container designed in the form of a trough;
[0073] FIG. 8 shows a fluid-filled folding bellows arrangement
disposed between a mirror and a base body;
[0074] FIG. 9 shows a fluidic control device formed by folding
bellows and arranged between a mirror and a base body;
[0075] FIG. 10 shows a mirror array having nine mutually
independently tiltable mirrors carried by piezo motors;
[0076] FIG. 11 shows a sectional representation of the mirror array
according to FIG. 10;
[0077] FIG. 12 shows a perspective representation of a piezo motor
according to FIGS. 10 and 11;
[0078] FIG. 13 shows an axial section through a mirror unit having
a liquid-filled movement gap;
[0079] FIG. 14 shows a plan view of a base body of the mirror unit
shown in FIG. 13;
[0080] FIG. 15 shows an axial section through a mirror according to
a variant of the mirror unit shown in FIG. 13;
[0081] FIG. 16 shows a view from below of the mirror shown in FIG.
15.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0082] FIG. 1 shows in perspective representation a detail of a
mirror unit 10, which is contained in an illumination system of a
microlithographic projection exposure apparatus. The detail shown
reveals a base plate 12 and a mirror unit that has a mirror 14,
which is held on a T-shaped support body 16, and two groups of leaf
springs 18 which are connected to the support body 16 and the base
plate 12 and consist of a material with high thermal conductivity,
for example steel, silicon, silicon carbide, copper, silver or
gold. Together with the support body 16, the leaf springs 18 form a
solid-state articulation for the mirror 14.
[0083] The mirror unit furthermore has two magnet coils 22 arranged
between the leaf springs 18 and the longer limb of the support body
16. Of course, other actuators may also be used instead of the
magnet coils. The mirror unit includes a plurality of the mirror
units represented in FIG. 1, for example several hundred or even
several thousand of them, which are arranged on the common base
plate 12. The base plate 12 may also be curved, so that the mirrors
14 arranged next to one another likewise form a curved common
mirror surface which is interrupted by the intermediate spaces
between the mirrors 14.
[0084] In the embodiment shown, the leaf springs 18 arranged on the
two sides of the support body 16 are respectively aligned mutually
parallel. In other embodiments, the leaf springs 18 engage on each
side of the support body 16 along a single line, so as to provide a
fanlike arrangement in which the leaf springs 18 then only
approximately extend mutually parallel.
[0085] The length of the longest limb 20 of the support body 16 is
adapted to the length 28 and the angular alignment of the leaf
springs 18 so that the support body 16 does not touch the base
plate 12 either in the neutral setting of the mirror 14, as
represented, or in an excursion setting (not shown). By
electrically driving at least one of the two magnet coils 22, a
force can be induced on the limbs 20 configured as permanent
magnets, so that they bend. Owing to the effect of the leaf springs
18, the induction of force leads to a swiveling movement of the
support body 16 about a swivel axis 24 which is shown. The leaf
springs 18 arranged on one side of the support body 16, which may
be regarded as firmly clamped in respect of their connection to the
support body 16 and the base plate 12, then become locally curved
and elastically deformed. On the opposite side, the leaf springs
act essentially as tensile elements which are stressed in tension
while being elastically deformed only slightly. The resistive
moment against bending, generated by the leaf springs 18 in a plane
of bending (not shown) is relatively small owing to the
dimensioning of the thickness 26 of the leaf springs 18. The plane
of bending is defined as a plane in which the leaf springs 18 are
capable of bending. Consequently, the plane of bending is arranged
perpendicularly to the swivel axis 24. In this plane of bending the
leaf springs 18 are spaced apart from each other, as can be clearly
seen in FIG. 1.
[0086] The dimensions and the number of the leaf springs 18 are
selected so that the product of thickness 26, width 30 and number
of the leaf springs 18 leads to a total cross section which is
significantly greater than that of solid-state articulations, which
instead of leaf springs 18 use elements that are monobloc but have
the same bending strength as the leaf springs 18. Owing to the
greater total cross section, the leaf springs 18 can dissipate more
heat from the mirror 14 in the direction of the base plate 12 and
thereby counteract overheating of the mirror 14. The subdivision of
hitherto monobloc elements of solid-state articulations into a
plurality of leaf springs 18 or other articulation elements
exploits the fact that the bending strength of a component is
reduced when it is subdivided into a plurality of individual parts,
but the heat flux remains the same. By increasing the cross section
of an articulation element, it is therefore possible to increase
the heat flux which it can transport but to keep the bending
strength constant owing to the subdivision into a plurality of
articulation elements.
[0087] In this embodiment, as well as in the embodiments explained
below, the base plate 12 may be provided with additional devices
such as cooling fins or cooling channels in order to be able to
dissipate better the heat absorbed from the mirrors. In addition or
as an alternative, the base plate may be thermally coupled to a
temperature sink.
[0088] The embodiment of a mirror unit 110 according to FIG. 2 uses
the references as in FIG. 1 respectively increased by 100 for
functionally equivalent components, which also applies
correspondingly for the further embodiments.
[0089] The leaf springs 118 according to FIG. 2 have a smaller
width 130 in the direction of the swivel axis 24, compared with the
leaf springs 18 according to FIG. 1. In order to ensure the desired
stability of the solid-state articulation formed by the leaf
springs 118, the number of leaf springs 118 is increased
significantly compared with the number of leaf springs 18 according
to FIG. 1. Five adjacently arranged groups, each of four leaf
springs 118 aligned mutually parallel, extend on each side of the
swivel axis 24. The total cross section and therefore the
transportable heat flux of the leaf springs 118 is increased
further in relation to the embodiment shown in FIG. 1, but without
thereby modifying the bending strength significantly.
[0090] FIG. 3 schematically represents a mirror unit 210 which
includes a base plate 212, a cuboid mirror 214 and a flexion
element 232, which is designed as a piezoelectric movement
transducer. By applying electrical potentials to electrodes (not
shown) of the flexion element 232, an excursion of the mirror 214
can be induced from a neutral position (not shown) into an
excursion position represented in FIG. 3. So that the flexion
element 232 can induce a sufficiently large swiveling angle for the
mirror 214, it has a significantly smaller cross section in
relation to the extent of the mirror 214. As a result of this cross
section, only a part of the heat released by absorption of
radiation in the mirror 214 can be dissipated into the base body
212.
[0091] In order to avoid overheating of the mirror 214, the mirror
unit has metal filaments 234, first ends of which are thermally
conductively connected to the mirror 214 (preferably in the
vicinity of the circumference) and second ends of which are
thermally conductively connected to the base plate 212. The metal
filaments 234 allow dissipation of heat from the mirror 214 to the
base plate 212. The diameter of the metal filaments 234 is so small
that they have a high flexibility, i.e. negligible elastic
properties. The metal filaments 234 therefore oppose swiveling
movements of the mirror 214 with only a small resistance, which can
readily be overcome by the flexion element 232. In order to improve
the thermal coupling to the mirror 213 and the base plate 212, in
this embodiment the metal filaments 234 are fastened on metal
strips 235 which in turn are fitted flat on the mirror 213 and the
base plate 212, respectively.
[0092] Instead of metals such as copper, silver or gold, it is also
possible to use silicon, silicon compounds, in particular silicon
carbide, or carbon as filamentary thermal conduction elements. The
thermal conduction elements may also be in the form of bands or
have other cross sections, so long as sufficient flexibility is
ensured.
[0093] In the mirror unit 310 according to FIG. 4, the mirror 314
is mounted so that it can be moved by swiveling via a bearing
element 336 in the form of a spherical segment, which is held in a
spherical cap-shaped recess of a bearing block 338. As a device for
controlling the mirror 314 relative to the base plate 312, bars
340, 342 are provided which mutually engage in a comb-like fashion,
are respectively fastened on the base plate 312 and on the mirror
314 and are divided (not shown) into four circular quadrants each
with an angular extent of approximately 90 degrees. If swiveling
can take place about only one swivel axis, then the bars 340, 342
may be in the form of cylinder wall segments as shown in FIG. 4. In
the event that swiveling can take place about two orthogonal axes,
the bars 340, 342 should either be very short or have no curvature
with respect to the swivel axes.
[0094] The bars 340, 342 are respectively equipped with
electrically insulating coatings (not shown), so that an
electrostatic drive can be formed by applying different potentials
to the bars 340, 342 arranged in the quadrants. By modifying the
applied electrical potentials, this drive allows swiveling movement
of the mirror 314 about two swivel axes aligned mutually
perpendicularly. The bars 340, 342 also have the function of
transferring the heat released by absorption of radiation from the
mirror 314 to the base plate 312.
[0095] In the embodiment shown in FIG. 4, in which swiveling can
take place about only one swivel axis, the bars 340, 342 may touch
irrespective of the alignment of the mirror 314 so that they can
transfer heat directly, i.e. via thermal conduction in solids, from
one bar 340 to the adjacent bar 342. Owing to the multiplicity of
relatively small bars 340, 342, a large surface area is available
which can be used for heat transfer. In general, however, narrow
gaps whose width may be less than 1/10 of the maximum dimension of
the reflective surface of the mirror 214 remain between the bars
340, 342. The heat transfer then takes place through the gas
molecules contained in the gap. Given a sufficiently small gap
width, a large heat flux is possible even when the gas pressure is
very low, as is the case in EUV illumination systems. The advantage
of bars sweeping along one another without touching is primarily to
avoid frictional losses, which otherwise would involve higher
control forces and correspondingly more elaborate driving.
[0096] The mirror unit 410 having two mirrors 414, as represented
in FIG. 5, has a similar structure to the mirror unit 210
represented in FIG. 3. In contrast to the mirror unit 210 according
to FIG. 3, fluid cooling of the lower sides of the mirrors 414 is
provided in the mirror unit 410 according to FIG. 5. In order to
prevent the coolant from escaping into the radiation space lying
above the mirrors 414, peripheral regions of the mirrors 414 are
respectively connected to flexible sealing elements 444 which are
fastened on neighbouring mirrors 414 or on wall regions 446. The
sealing elements 444, made of thin-walled metal foil, allow mutual
relative movement of the mirrors 414 and, together with the mirrors
414 and the base plate 412, delimit a closed volume section 445 in
which a coolant can flow. The volume section may also be filled
once with the coolant, which remains there permanently or over a
prolonged period of time.
[0097] The coolant may be a liquid, for example mercury, water or
gallium-indium-tin. In order to increase the thermal conductivity,
metal particles may also be added to the liquid.
[0098] It is, however, also conceivable to use a gas as the
coolant. The sealing elements 444 will then isolate the gaseous
coolant placed at high pressure in the volume section 445 from the
protective gas (which above all in EUV systems is at a very low
pressure) that fills the volume on the other side of the sealing
elements 444. So that the forces on the mirrors 414 and above all
the sealing elements 444 do not become too large, however, the
pressure difference between the two gases adjacent to the sealing
elements 444 should not be too great. Yet since the thermal
conductivity of gases increases approximately linearly with
pressure at low pressures, even increasing the pressure by one or
two powers of ten is sufficient to increase the thermal
conductivity significantly.
[0099] The use of gaseous coolants is advantageous because flowing
or stationary gases are simpler to manage than liquids.
Furthermore, gaseous coolants create smaller frictional losses when
tilting the mirrors 414. Liquid coolants, on the other hand,
usually have better thermal conduction properties.
[0100] The mirror unit 510 according to FIG. 6 includes a bearing
element 548 designed in the form of a trough in which are arranged
two piezo controllers 550 aligned mutually parallel and engaging on
surfaces of a bar-like extension of the mirror 514, which face away
from one another, and a flexible metal bellows 552 provided for
exerting a tensile force on the mirror 514. A lower side of the
mirror 514 and corresponding glide surfaces of the bearing element
548 are respectively designed in the form of cylinder segments and
allow the mirror 514 to swivel in the plane of the drawing
according to FIG. 6. The swiveling movement is induced by applying
electrical potentials to the piezo controllers 550, the
longitudinal extent of which can be modified in the direction of
the arrows indicated according to the applied electrical potentials
so that a corresponding torque is exerted on the mirror 514.
[0101] In the neutral setting of the mirror 514 as represented, the
metal bellows 552 is prestressed downwards in the axial direction
and therefore pulls the mirror 514 into the bearing element 548.
Owing to the design of the metal bellows 552, it can jointly
execute the swiveling movement of the mirror 514 in the plane of
the representation according to FIG. 6, without thereby building up
undesirably high restoring forces. The metal bellows 552 can be
filled with a pressurised fluid through a liquid gland (not shown),
so that the prestress of the metal bellows 552 is neutralised and a
pressure force can be exerted on the mirror 514. This leads to the
movement gap 554 clearly visible in FIG. 6 between the mirror 514
and the bearing element 548. When carrying out a control movement
in the presence of the movement gap, virtually friction-free
adjustment of the mirror 514 can be performed. After the end of the
control movement, the pressurisation of the metal bellows 552 is
reduced so that the mirror 514 returns to resting on the bearing
element 548 in such a way as to transfer heat.
[0102] The movement gap 554 is preferably filled with a liquid or
paste (not shown), for example an electro- or magnetorheological
liquid, a thermally conductive paste, a vacuum grease or an oil, in
order to improve the heat transfer between the mirror 514 and the
bearing element 548. The liquid compensates for surface roughness
or fitting mismatches, and therefore avoids heat transfer being
able to take place only through gas inclusions. For the same
purpose, the mutually opposing surfaces of the mirror 514 and of
the bearing element 548 may be made of a soft but thermally
conductive material, for example indium, aluminium or copper, or
provided with an inlay consisting of such a material. Coating with
DLC (diamond-like carbon) also has a favourable effect on the
achievable dissipation of heat.
[0103] The mirror unit 610 represented in FIG. 7 has a similar
structure to the mirror unit 310 represented in FIG. 4. However,
the bars 640, 642 shaped like spherical sleeves are merely used for
heat transfer between the mirror 614 and the base plate 612. An
array of four flexion elements 632 arranged to form a square is
provided as the control device, which allows a swiveling movement
of the mirror 614 in two mutually perpendicular spatial directions.
Lateral wall regions 646 are arranged on the base plate 612, which
together with the base plate 612 and the mirror 614 delimit a fluid
space 656 through which a coolant, for example a cooling gas such
as hydrogen or a liquid such as mercury, gallium-indium-tin or
water, can be fed. For supplying and discharging the coolant,
connection glands 658 are arranged in the base plate 612. Suction
glands 660 are also provided in the wall regions 646 above the
mirror 614, which make it possible to suction coolant that may
escape from the fluid space 656 through sealing gaps 654 and could
lead to degradation of the optical properties in the vicinity of
the mirror 614.
[0104] In the mirror unit 710 according to FIG. 8, two folding
bellows 762 are arranged between the mirror 714 and the base plate
712, which are filled with a liquid coolant and therefore improve
heat transfer between the mirror 714 and the base plate 712. The
flexion element 732 provided for swiveling the mirror 714 is
equipped with a through-bore 764, which allows fluid exchange
between the two folding bellows 762.
[0105] Instead of the through-bore 764, a channel 766 extending
through the mirror 714 may also be provided for the purpose of
fluid exchange as indicated by dashes in FIG. 8. In another
variant, the coolant is circulated in the folding bellows 762 in
order to permit even better dissipation of heat. To this end, the
folding bellows 762 should be provided with inlet and outlet glands
for the coolant (not shown).
[0106] Two separate folding bellows 862 are provided in the mirror
unit 810 according to FIG. 9, which can be supplied with a
pressurised fluid through respectively allocated liquid glands 866.
Each of the liquid glands 866 is allocated an electromagnetic
linear motor 868 which includes a permanent magnet 870 held
linearly mobile in the liquid gland 866 and a coil 872, which is
arranged coaxially with the permanent magnet 870 and to which an
electrical voltage can be applied. Since the fluid volume in a
folding bellows 862 and the respectively allocated liquid gland 866
is closed, a translational movement of the permanent magnet 870
leads to a volume variation in the liquid gland 866 which is
compensated for by an opposite volume change in the folding bellows
862. The volume changes in one of the folding bellows 862 lead to
tilting of the mirror 814.
[0107] The mirror unit 910 represented in FIGS. 10 to 12 includes a
base plate 912 on which a total of nine tilting drives 974 are
arranged, which allow tilting of allocated mirrors 914 in two
mutually orthogonal spatial directions. Below the base plate 912, a
heat sink 976 made of a material with high thermal conductivity is
arranged, which has flow channels 978 for a coolant. Each of the
tilting drives 974 has four ultrasonic transducers 980, which
respectively have a square across section and are grouped so that,
apart from movement gaps 982 remaining between neighbouring
ultrasonic transducers 980, they form an array with a square cross
section.
[0108] Each of the ultrasonic transducers 980 is designed as a
piezoelectric flexion element. Respectively opposite ultrasonic
transducers 980 can be deformed in a common bending plane 984, the
bending planes of neighbouring ultrasonic transducers 980 being
aligned mutually perpendicularly. An indentation 986 in the form of
a spherical cap segment is respectively provided on an upper side
of the ultrasonic transducers 980; the indentations 986 of the four
ultrasonic transducers 980 which form a tilting drive 974 add
together to form a virtually hemispherical indentation. On mutually
opposing inner surfaces of the ultrasonic transducers 980, recesses
are respectively provided in the form of conical segments which add
together in the ultrasonic transducers 980 arranged to form a
square to create a free space in the shape of a conical
segment.
[0109] The mirror 914 is fitted on an axisymmetric support body
916. The latter has a region 990 in the form of a spherical
segment, next to which there is a region 992 in the form of a
conical segment. The region 990 in the form of a spherical segment
rests flat on the surfaces of the indentations 986 of the
ultrasonic transducers 980, as represented in detail in FIG. 11.
The region 992 in the form of a conical segment is arranged in a
free space formed by recesses 988 of the ultrasonic transducers
980, so that the support body 916 can be tilted in two mutually
perpendicular spatial axes.
[0110] On the end of the region 992 in the form of a conical
segment, a metal bellows 952 is arranged which is likewise designed
axisymmetrically and is fastened on the heat sink 976 at an
opposite end from the support body 916. Owing to the axisymmetric
design of the metal bellows 952, it is rendered flexurally elastic
and torsionally stiff so as to allow the support body 916 to swivel
in two mutually perpendicular spatial directions, while rotation of
the support body about the longitudinal mid-axis 994 is prevented
by the torsional stiffness of the metal bellows 952. Making the
bellows 952 from metal ensures that there are only parts which have
a high thermal conductivity between the mirror 914 and the heat
sink 976. If a particularly high thermal conductivity of the
bellows 952 is desired, then nickel in particular may be envisaged
as the metal. If however minimal stiffness is paramount as a
selection criterion, then titanium may be suitable as the material
for the bellows 952.
[0111] In order to tilt the mirror 914 about a swivel axis lying in
the mirror surface, driving of mutually opposite ultrasonic
transducers 980 is respectively provided. The oppositely arranged
ultrasonic transducers 980 will be driven so that they bend at
least essentially synchronously and in the same direction. This
induces a tilting movement of the support body 916. The mutually
opposite ultrasonic transducers 980 will then be driven so that
they bend with slight shortening in the opposite direction. At this
time, there is no contact of the correspondingly driven ultrasonic
transducers 980 with the support body 916. Subsequently, by
appropriate driving of the ultrasonic transducers 980, renewed
contact with the support body 916 is established and the mutually
opposite ultrasonic transducers 980 can again be deformed in the
desired direction by once more applying electrical energy. A
stepwise tilting movement of the support body 916, and the mirror
914 arranged on it, therefore takes place overall. Owing to this
relative mobility of the ultrasonic transducers 980 with respect to
the support body 916, the maximum swivel angle for the mirrors 914
is restricted merely by the design geometry of the interacting
components. A suitable configuration can allow tilting of the
mirror 914 in the range of up to +/-15 degrees.
[0112] In the neutral setting as represented in FIG. 11, the metal
bellows 952 is already prestressed in the axial direction and
therefore exerts a tensile force on the support body 916, so that
the latter rests flat in the indentations 986 of the ultrasonic
transducers 980. In order to improve the dissipation of heat from
the mirrors 914 in the direction of the heat sink 976, a fluid,
preferably a cooling gas, may flow through the movement spaces
provided in the region of the metal bellows 952 and between the
support body 916 and the ultrasonic transducers 980. In this case,
the movement gaps 982 between the ultrasonic transducers 980 may be
closed by elastic sealing materials so as to create a closed fluid
channel from which the fluid cannot escape in the direction of the
mirrors 914. A fluid may also flow through the metal bellows 952
itself, in order to further increase the thermal conduction between
the mirror 914 and the heat sink 976.
[0113] FIGS. 13 and 14 show a mirror unit 1010 according to another
embodiment, respectively in an axial section and a plan view of a
mirror support body 1012 of the mirror array 1010. The mirror
support body 1012, which may be connected to a base body carrying a
plurality of mirror units or formed integrally thereon, has a
recess 1036 in the form of a spherical cap that corresponds with a
bearing element 1038 in the form of a spherical segment, which is
fastened on the mirror 1014 or is formed integrally thereon.
Between the mutually opposing curved surfaces of the mirror support
body 1012 and of the bearing element 1038, a movement gap 1054
through which a liquid (not shown in detail) flows is left during
operation of the projection exposure apparatus. To this end, in the
embodiment represented in which swiveling can take place about two
swivel axes, a total of five exit channels 1066 and four entry
channels 1067 are incorporated in the mirror support body 1012. The
exit channels 1066 open in the region of the recess 1036 into a
central exit opening 1058a and into four off-centre exit openings
1058b. The entry openings 1059 for the four entry channels 1067 lie
outside the recess 1036 in this embodiment.
[0114] Arrows in FIG. 13 indicate the flow direction of the liquid
in the movement gap 1054.
[0115] The liquid flows out of the central exit opening 1058a and
the off-centre exit openings 1058b, fills the movement gap 1054
uniformly and finally flows a way over the circumferential edge of
the recess 1036 in order to re-enter the mirror support body 1012
through the entry openings 1059.
[0116] Owing to the liquid contained in the movement gap 1054,
better heat transfer from the mirror 1014 to the mirror support
body 1012 is achieved than would be the case with the mutually
corresponding curved surfaces sliding directly on one another.
Furthermore, the liquid functions as a heat exchange medium which
absorbs heat from the mirror 1014 and dissipates it through the
entry channels 1067 to a thermal sink (not shown). By suitable
selection of the liquid and the ambient conditions, it is
furthermore possible to make the liquid partially evaporate and
thereby cool the mirror 1014. Evaporated liquid may be suctioned
with the aid of additional suction openings (not shown in FIGS. 13
and 14). This will prevent evaporated liquid from entering regions
through which projection light passes and degrading the optical
properties of the illumination system.
[0117] The thin liquid film in the movement gap 1054 furthermore
reduces the friction in a similar way to a lubricant, when the
mirror 1014 is being swiveled relative to the mirror support body
1012.
[0118] In this case, it may be favourable to prestress the mirror
114 relative to the mirror support body 1012. The prestressing may
be generated contactlessly, for example under the effect of
magnetic forces, or with the aid of elastic elements such as
springs or bellows.
[0119] Such bellows may also be used as a seal, in order to
reliably prevent the liquid emerging from the gap between the
mirror 1014 and the mirror support body 1012. Preferably, in this
case, the bellows hermetically enclose all regions conveying the
liquid, i.e. the movement gap 1054, the entry openings 1059 and the
exit openings 1058a, 1058b.
[0120] In another embodiment, entry openings 1059 are also arranged
inside the recess 1036, for example next to its circumferential
edge. In order to prevent the liquid from flowing away over the
upper edge of the recess 1036, a ring-like bar concentrically
enclosing the recess 1036 may be formed on the plane upper side of
the mirror support body 1012. The bar locally reduces the width of
the movement gap 1054, so that it is not so easy for the liquid to
escape from the movement gap 1054.
[0121] It is, however, also conceivable to provide an annular entry
opening, preferably centred with respect to the recess 1036, so
that the liquid can be discharged over a large area.
[0122] The liquid flowing in the movement gap 1054 may also be used
to induce swiveling movements of the mirror 1014 relative to the
mirror support body 1012. This will utilise the friction between
the flowing liquid and the bearing element 1038, which leads to a
torque on the mirror 1014. In order to reinforce this effect,
measures may be instigated in order to increase the drag of the
surface of the bearing element 1038.
[0123] FIG. 15 shows an axial section of a variant, denoted by
1014', of the mirror 1014 shown in FIG. 13. In this variant the
bearing element 1038' formed on the mirror 1014' is provided with
structures 1070 to increase the drag on its curved surface. The
structures 1070 may for example be fine ribs, which extend
transversely to the flow direction of the liquid and thereby
generate an increased drag.
[0124] An expedient arrangement of such rib-like structures 1070 is
shown in the view from below of the mirror 1014' presented in FIG.
16. If a liquid flows out only from one of the off-centre entry
openings 1058b, then the emerging liquid will sweep over the
structures 1070 and generate a torque on the mirror 1014', which
causes it to swivel. In order to swivel the mirror back again,
liquid will be introduced into the movement gap 1054 (exclusively)
through the entry opening 1058b respectively lying diametrically
opposite. By modifying the flow direction of the liquid in the
movement gap 1054, it is therefore possible to exert forces on the
mirror 1014' which lead to swiveling about the desired swivel
axis.
[0125] It is to be understood that a gas may also be used instead
of a liquid in the embodiments shown in FIGS. 12 to 15.
* * * * *